Some embodiments of the present invention relate to analog-to-digital conversion and image sensors and, more particularly, to methods and systems for analog-to-digital conversion applicable to an image sensor, such as a CMOS image sensor, in which pixels can be readout non-destructively.
In some image sensors, conditional reset of a pixel is used to increase dynamic range of the pixel. For instance, a first non-destructive read of the pixel is performed to determine if the pixel signal has exceeded a threshold. If this conditional reset is met (i.e., the pixel output exceeds the threshold), then the pixel output value would be converted to a digital value that would be stored, and the pixel would be reset to restart integration.
If the conditional reset is not met (i.e., the pixel output does not exceed the threshold), then the pixel would continue to integrate as-is, without conversion (and storage) of the pixel value from the first non-destructive read. That is, the pixel would not be reset before being subsequently read. In some image sensors, only one such subsequent read during the same integration time may be permitted (i.e., the pixel output value from this one subsequent read would be converted to a digital value that would be stored, and the pixel would be reset to restart integration), whereas some image sensors may be designed to permit more than one possible subsequent non-destructive read (e.g., until the reset condition is met, or a certain number of reads is performed, whichever occurs first) before conversion of the pixel value, storage of the digital value, and resetting the pixel to restart the integration period for the pixel.